In computing, multi-touch refers to a touch sensing surface's ability to recognize the presence of two or more points of contact with the surface. This plural-point awareness is often used to implement advanced functionality such as pinch to zoom or activating predefined programs (Wikipedia, “multi-touch”). The Windows 8 operating system from Microsoft Corporation requires a touch screen supporting a minimum of 5-point digitizers. WINDOWS® is a registered trademark of Microsoft Corporation.
The present invention relates to light-based touch sensitive surfaces. Light-based touch sensitive surfaces surround the surface borders with light emitters and light detectors to create a light beam grid above the surface. An object touching the surface blocks a corresponding portion of the beams.
Reference is made to FIG. 1, which is a diagram of a prior art, light-based touch screen having 16 LEDs and 16 PDs. Screen 801 in FIG. 1 is surrounded by emitters 101 along two edges and photodiode (PD) receivers 201 along the remaining two edges, which together emit and receive a lattice of light beams 300 covering the screen.
Light-based touch detection systems are unable to accurately recognize many instances of two or more points of contact with the surface. Reference is made to FIGS. 2 and 3, which are illustrations of instances of ambiguous multi-touch detections in prior art touch screens. FIGS. 2 and 3 show different instances of two, diagonally opposed touches 901 and 902 that are ambiguous vis-à-vis the light grid of FIG. 1. As shown in FIGS. 2 and 3, the same light beams are blocked in both instances.
There is further ambiguity when more than two objects touch the screen simultaneously. Reference is made to FIGS. 4 and 5, which are illustrations of instances of ghosted touches in prior art touch screens. The two-touch cases shown in FIGS. 2 and 3 are also ambiguous vis-à-vis the three-touch case, 901-903, shown in FIG. 4, and vis-à-vis the four-touch case, 901-904, shown in FIG. 5. In each of the cases illustrated in FIGS. 2-5, row and column PDs a-h show an absence of light in the same locations. The ambiguity illustrated in FIGS. 4 and 5 is caused by “ghosting”, which refers to an effect where the shadow of a first object obscures a second object and prevents the second object from being detected.
Reference is made to FIG. 6, which is an illustration of a prior art method of disambiguating multi-touch detections in prior art touch screens. The real touches shown in FIG. 6 are at locations A and B. They both block light in the X and Y directions, and this is detected on the right and bottom edges. However, it is possible to add touches at locations C and D without any effect to the light blocking pattern. With the available data it is not possible to determine what the real touch points are, and whether there are in fact 2, 3 or 4 of them.
As shown in FIG. 6, prior art methods of disambiguating multi-touches rely on additional light beam directions, to create a less ambiguous blocking pattern. For example, adding diagonally-oriented beams (arrow 1) to the scenario in FIG. 6 reveals that C and D are not touch locations because diagonal beams would pass unblocked. On the other hand, if there were really three touches, say, A, B and C, it would not be possible to distinguish whether the third touch is at C or D or both. In order to do so, a second diagonal direction is added (arrow 2).
FIGS. 1-6 are simplified in that they show touch sizes that are uniform and locations that are neatly aligned with the light beams. In reality, though, some touches may partially block more than one beam while still shadowing each other. Even with a large number of beam directions some touch configurations generate a blocking pattern with ambiguous or surplus touch location candidates. Furthermore, the detection data is not perfect which makes it difficult to perform direct geometric calculations on the touch locations.
Reference is made to FIG. 7, which is a prior art blocking pattern of light intensity from measured five-touch data. Each blocked light beam has been shadowed according to its blocking level. It is easy to see, based on the blocking pattern, that there could be more than ten touches. The real touches are not even placed on the most blocked locations. Had they not been highlighted in FIG. 7, it would have been impossible to pinpoint them from the blocking pattern.
The present invention also relates to integrated casings for optical touch screen displays in which the outer casing, or housing, for the display is a light guide for guiding light from emitters mounted within the housing over and across the screen surface, and for guiding beams that have crossed the screen surface to light receivers mounted within the housing.